Understanding frame geometry

General cycling advice ( NOT technical ! )
martinn
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Understanding frame geometry

Postby martinn » 7 Jan 2018, 7:30pm

Hi all
I am sure this has come up before, but I couldnt find what I wanted on a search.

I have read a few articles on frame geometry, but I still don't quite get it!
I was looking at getting a new bike with disc brakes, and thought I would go Ti at the same time. I have looked at numerous bikes and geometry, trying to compare them to see what size I would need. Now I do realise that the only way to really see if a bike fits it to get on and ride it. Unfortunately this would be challenging to achieve due to the location of many of the stockists/ makers.
So I measured two bikes that I currently ride one is more comfortable Thant the other and they are different sizes. HTT, but have the same bb to saddle height.
So for fit as opposed to ride characteristics, what if any measurement is most important. Ie where you would start from?

Many thanks
Martin

tatanab
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Re: Understanding frame geometry

Postby tatanab » 7 Jan 2018, 8:20pm

martinn wrote: HTT
It took me some time to figure that one out. I came up with a couple of possibilities before I realised it probably means Horizontal Top Tube.

To the question - I pretty much ignore frame geometry and set my machines so that the pedal to saddle distance is the same (not bottom bracket to saddle because I might have different length cranks on different machines), then saddle peak to handlebar centre and the saddle peak to handlebar drop. I have ridden frames in the range 21" to 24" when need be through finances, but have the same riding position.

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NATURAL ANKLING
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Re: Understanding frame geometry

Postby NATURAL ANKLING » 7 Jan 2018, 9:23pm

Hi,
It used to be that frames were measured at seat tube *BB centre to say top of seat tube.
You could add BB to top of Top Tube, where seat tube joins top tube.
Or even BB to top tube and seat tube intersection centre, measured along the centre of seat tube.
Probably the most useful would the second option. But that was when top tubes were horizontal.

Nowadays the fashion is slopping top tubes, but "Virtual Top Tube" or "Effective Top Tube" might well be quoted as well as real top tube length, real top tube is superfluous with a sloping Top Tube!
This is a good measure of actual frame size for all bike frames.
Top Tube sets reach basically.
You just need to keep an eye on seat tube, and make sure that with a seat post and the seat tube length you can get your own personal BB to saddle height. - That's Real Seat Tube! Virtual / Effective means nothing.

Actual / Real seat tube length quoted + saddle pillar length (measured along plain inserted portion) will give 2" or 50MM minimum insertion of seat pillar into frame. Allowing 50 - 75 MM for saddle height from rails.
So - 500 MM seat tube plus a 200MM saddle pillar will give 700 MM saddle (Top) height above BB centre but only 50 MM minimum insertion of pillar in seat tube.
Recommended minimum insertion will be between 75 - 100 MM for safety (see markings on saddle pillar)
Saddle pillar / posts come up to 400 MM, some can be got at 500 MM.
So - Real seat tube + saddle pillar (plain length of pillar) length - 50 MM will give a safe insertion of saddle pillar of 100 MM.
EG - 550 MM frame seat tube + 200 MM saddle pillar - 50 MM = 700 MM BB to Top of saddle , Measured along the seat tube!

Measure your Top Tube Virtual / Real on horizontal TT only, on your most comfortable bike.
Then check Real seat tube length and your personal BB to saddle top, to make sure you can get a saddle pillar to suit.
MTB frames have short real seat tubes so beware...............

https://geometrygeeks.bike/understanding-bike-geometry
"Top Tube Length
aka: Effective Top Tube, Horizontal Top Tube, Virtual Top Tube
Top tube is a good indicator of overall size of the bike.
It's measured 'effectively' horizontally from head tube axis to seat tube axis.
Older bikes frequently had horizontal top tubes, but now sloping tubes are much more common. Where the top tube is horizontal the Effective Top Tube and Actual Top Tube will be identical.
A longer top tube has you more stretched out on the bike, all other things being equal. This will give you a racier, more aerodynamic position on a road bike - possibly at the expense of all-day comfort.
In mountain biking, top tubes have been lengthening as stems get shorter and bars wider, giving a more stable ride at speed."
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Brucey
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Re: Understanding frame geometry

Postby Brucey » 7 Jan 2018, 9:23pm

it will take little while to measure up your extant bike, and then enter the measurements of any new frame, but it might save you a lot of grief if you use this tool

https://www.bikecalculator.co.uk/

or similar.

There isn't really "an unimportant dimension" in a bike frame. At a bare minimum you want to be able to fit onto the bike comfortably, but there is potentially a lot more to it than that.

cheers
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531colin
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Re: Understanding frame geometry

Postby 531colin » 7 Jan 2018, 9:39pm

martinn wrote:............
So for fit as opposed to ride characteristics, what if any measurement is most important. Ie where you would start from?........


Have you reached the limit of adjustment in any respect with your current bikes?
Reading this forum suggests that many people struggle to get their bars high enough, so a tall head tube or a big "stack" value would be appropriate, or room for lots of headset spacers.
If your saddle is currently as far back as it will go, then you should probably seek a slack seat tube angle.

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horizon
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Re: Understanding frame geometry

Postby horizon » 7 Jan 2018, 11:18pm

The OP is trying to buy the right size bike. But that's the easy bit: there will only be three or four sizes and he could probably go with either one of two of them. The rest is fit - tweaking at home using the stem, saddle, cranks etc.

However there are limits which I suppose is why 531colin asks:

Have you reached the limit of adjustment in any respect with your current bikes?


The answer for most people is no. But I have found that increasing saddle layback is a problem (solution = VK adaptor, not everyone's choice), decreasing reach (solution = shorter stem which still may not be quite enough), increasing handlebar height (solution = stem raiser, again an ugly addition).

So what the OP needs to do is look carefully less at the actual size in the range but two things IMV (at least in my own case): the real reach (effective top tube plus or minus the seat angle) and head tube height (if the steerer is already cut short then the dealer needs to be put out of his misery).

Personally I think these two things will determine the starting point on every type of bike (racing, touring etc) but I'm open to other views on this.

PS I was thinking this through as I wrote, trying to draw conclusions so I'm very open to being corrected.
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elPedro666
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Re: Understanding frame geometry

Postby elPedro666 » 8 Jan 2018, 10:05pm

Far and away the most important two dimensions in my opinion are reach and stack* - other opinions are of course available. These are the the vertical and horizontal distances from the centre of the BB to the centre of the top of the headtube. Everything else is adjustable with varying consequences but this determines where you stand within the bike and if correct means you can select your stem length for the handling characteristics you desire, and your saddle set back for your comfortable pedalling position, rather than balancing those two things to determine the reach.

As an aside, we generally get away with using top tube and seat tube measurements instead simply because seat angles don't, as a rule, vary all that dramatically.

*I think this may actually be Keith Bontrager's opinion which I have assimilated...

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francovendee
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Re: Understanding frame geometry

Postby francovendee » 9 Jan 2018, 8:34am

Brucey wrote:it will take little while to measure up your extant bike, and then enter the measurements of any new frame, but it might save you a lot of grief if you use this tool

https://www.bikecalculator.co.uk/

or similar.

There isn't really "an unimportant dimension" in a bike frame. At a bare minimum you want to be able to fit onto the bike comfortably, but there is potentially a lot more to it than that.

cheers


That's a useful tool, I now understand where all the measurements are taken from. Thanks

PeterBL
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Re: Understanding frame geometry

Postby PeterBL » 9 Jan 2018, 11:47am

elPedro666 wrote:Far and away the most important two dimensions in my opinion are reach and stack* - other opinions are of course available.

Agreed.

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andrew_s
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Re: Understanding frame geometry

Postby andrew_s » 9 Jan 2018, 4:07pm

NATURAL ANKLING wrote:"Effective Top Tube" might well be quoted as well as real top tube length,...
This is a good measure of actual frame size for all bike frames.

That's the first thing I look at when considering frames these days.
I'd also look at saddle setback (from the BB - i.e. the difference between effective top tube and reach). I don't think reach on its own is too useful.

That Mike Burrows chap has a lot to answer for ;)

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531colin
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Re: Understanding frame geometry

Postby 531colin » 9 Jan 2018, 9:22pm

Reach is a useful dimension, but there are people who can't get their saddle far enough back with a seat tube angle much steeper than 72 degrees.......if you are in this group, you need to figure seat tube angle into the equation as well.
I think saddle setback is set by your weight distribution on the bike......you should be able to let go of the bars without falling forwards and chinning the bars https://www.stevehoggbikefitting.com/bikefit/2011/05/seat-set-back-for-road-bikes/.....skip down to "point of balance" if you're impatient.
Reach to the bars can be tuned by stem length.......contrary to myth, it needs a big change in hand position to influence steering feel......it doesn't feel different (to me) going from the tops to the hoods to the drops, but if you move back say four inches (going from drops to swept back bars with the same stem) you will feel the difference.

elPedro666
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Re: Understanding frame geometry

Postby elPedro666 » 9 Jan 2018, 10:17pm

531colin wrote:Reach to the bars can be tuned by stem length.......contrary to myth, it needs a big change in hand position to influence steering feel......it doesn't feel different (to me) going from the tops to the hoods to the drops



Possibly because I come from a predominantly offroad background I must take exception to this - I would say that anything over 10mm difference in stem length makes a profound difference to the handling characteristics. On smoother surfaces this is much less of an issue and you can adapt more readily, but it is still clearly noticeable.

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reohn2
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Re: Understanding frame geometry

Postby reohn2 » 9 Jan 2018, 11:33pm

I'm with Colin,from tops to hoods is a lot more than that 10mm,in fact 80mm reach from those two positions on the h/bar I use,the drops are behind the tops by about 20 to 30mm.I can't feel any difference in all three positions.
Flats are far wider than drops from 220mm upto 320+mm wider,could this be the reason stem length makes all the difference?
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Brucey
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Re: Understanding frame geometry

Postby Brucey » 10 Jan 2018, 1:10am

elPedro666 wrote:
531colin wrote:Reach to the bars can be tuned by stem length.......contrary to myth, it needs a big change in hand position to influence steering feel......it doesn't feel different (to me) going from the tops to the hoods to the drops



Possibly because I come from a predominantly offroad background I must take exception to this - I would say that anything over 10mm difference in stem length makes a profound difference to the handling characteristics. On smoother surfaces this is much less of an issue and you can adapt more readily, but it is still clearly noticeable.

Blindfold me and I'll prove it...
.


I think you can both be right; if you have the kind of riding position where you are nicely balanced most of the time and there isn't much weight on the handlebars, Colin is right. But... if you have the kind of riding position where there is much weight on the handlebars at times, stem length makes a big difference.

Note also that if you are set up (eg for racing) so that you are 'balanced' when going 10/10ths, there is some weight on the bars when you are just tapping along, and stem length is again important.

When I raced it made sense to set up my bike so that I was well balanced when producing several hundred watts. I can't do that any more.... -not for very long, anyway... so on the same bike I have to put up with more weight on my hands more of the time or I have to put my saddle back further.

cheers
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elPedro666
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Re: Understanding frame geometry

Postby elPedro666 » 10 Jan 2018, 9:40am

Brucey wrote:
elPedro666 wrote:
531colin wrote:...contrary to myth, it needs a big change in hand position to influence steering feel...



Blindfold me and I'll prove it...
.


I think you can both be right



You are our very own Yo-Yo Ma Brucey

I'm intrigued by the amount bar width plays into this, as suggested above. It definitely rings true to experience for me to some extent, but all the bikes with flat bars tend to be offroaders hence require significantly more steering input, whereas those with drop bars tend to be steered much more delicately so my experience is perhaps flawed or at least skewed. Fair to say that one or other or both of these factors play into stem length being much more significant offroad in terms of handling, and much more forgiving on a smooth surface - although I can still clearly feel the difference, it's much less significant on tarmac.

I think that may just have been a long winded way of moreorless agreeing with everyone

Offer of a blindfolded test still stands though

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